The invention relates to a high-frequency transistor including a semiconductor body of silicon having a highly-doped substrate of a first conductivity type, a more weakly-doped epitaxial layer of the first conductivity type disposed on it, a layer-shaped base region of the second, opposite conductivity type provided in the epitaxial layer and a pattern of silicon oxide at least in part countersunk into the epitaxial layer and subdividing the base region into a number of mutually separated base zones, which are interconnected by conducting layers located on the isolating pattern, at least one emitter zone of the first conductivity type being provided in each base zone.
The invention also relates to a method of manufacturing the device.
A high-frequency transistor of the kind described is known from the published Japanese Patent Application (Kokai) No. 56-94770.
High-frequency transistors are used for various purposes, more particularly as transistors for broadband antenna amplifiers and as transmitter transistors. They should satisfy a number of requirements. The most important requirements are that a high amplification is obtained within a wide frequency range and a long life is achieved (generally associated with a low thermal resistance).
These requirements can be satisfied by a transistor structure, which has, in addition to narrow strip-shaped emitter zones a low emitter-base and base-collector capacitance and further a low thermal resistance. The term "thermal resistance" is to be understood to mean the maximum temperature difference between the base-collector junction and the heat sink divided by the dissipated power.
However, these three requirements are contradictory to a certain extent. Starting from a transistor structure comprising a number of emitter zones or emitter "fingers" in a common base zone, with unchanged distance between the emitter zones and with unchanged emitter surfaces area, a reduction of the width (and hence an increase of the number) of the emitter zones will lead to a larger base surface area, which gives rise on the one hand to a lower thermal resistance, but on the other hand to a higher base-collector capacitance, efforts can then be made to reduce the base surface area and hence the base-collector capacitance by reduction of the distance between the emitter zones, but this again leads to an increase of the thermal resistance. A known method of reducting the thermal resistance consists in that the emitter zones are distributed over several mutually-separated base zones. As a result, however the base-collector capacitance again increases.
With the construction of a transistor according to the aforementioned Japanese Patent Application No. 56-94770, a layer-shaped base region is subdivided into mutually-separated base zones by a sunken oxide pattern. Thus, it is possible to space the base zones apart by an arbitrarily large distance and hence to obtain a low thermal resistance without increasing the base-collector capacitance.
However, a disadvantage of this transistor is that the emitter-base capacitance is high due to the fact that the emitter-base junction in the form of a planar p-n junction is bounded by highly doped base contact zones. A further disadvantage is that the manufacture of this transistor requires a fairly complicated technology, which inter alia comprises an underetching step. Such an under-etching step is often poorly reproducible, which is disadvantageous especially with the fine structure used in high-frequency transistors.